added front page samples to testing, plus a couple of tweaks to front page and code lab

src/tests/site/code/testing.dart

+import 'package:unittest/unittest.dart';
+import 'dart:math' show Point;
+
+main() {
+  Point a = new Point(2, 15);
+  Point b = new Point(7, 3);
+
+  test('point distances', () {
+    expect(a.distanceTo(b), equals(13));
+  });
+  test('point magnitude close to', () {
+    expect(a.magnitude, closeTo(15, .25));
+  });
+  test('point magnitude greater than', () {
+    expect(a.magnitude, greaterThan(b.magnitude));
+  });
+  test('point == operator', () {
+    expect(a == b, equals(false));
+  });
+}